The load supporting capacity of a concrete pile formed in an earthen shaft has been conventionally measured by placing a jack at the upper end of the pile, and supporting the reaction of the jack with earthen anchors and/or similar piles located around the concrete pile to be tested. The necessary downward load can be applied to the concrete pile by the jack, and the reaction acting on the jack can be supported by the resistance of the earthen anchors and/or the surrounding piles against the pulling force. To eliminate the need for the massive structure for supporting the reaction force produced by the jack, it was proposed to provide a hydraulic jack at the bottom end of the shaft, and supply hydraulic fluid from the ground surface. For instance, reference should be made to Japanese patent publication (kokoku) No. 48-27007 (Nakayama et al), Japanese patent publication (kokoku) No. 53-12723 (Sumii), and U.S. Pat. No. 4,614,110 issued Sep. 30, 1986 to Osterberg. By extending the hydraulic jack at the bottom of the shaft, it is possible to determine the end bearing capacity and the unit shaft resistance of the pile by measuring the displacements of the two ends of the hydraulic jack with respect to a fixed point of reference typically defined on the ground surface.
It was also proposed to provide a hydraulic jack in a middle part of the pile to more accurately evaluate the bearing capacity, in particular the unit shaft resistance of the pile. For instance, reference should be made to Japanese patent publication (kokoku) No. 6-63879 (Arai et al).
According to this previously proposed device, the hydraulic jack provided in an intermediate part of the pile creates a certain discontinuity. More specifically, because the central part of the hydraulic jack is closed, it is necessary to build the part of the pile below the hydraulic jack by first placing a steel cage for concrete reinforcement or a rebar cage in the bottom of the shaft, and placing concrete in this bottom region before installing the hydraulic jack. The same process is repeated for constructing the part of the pile above the hydraulic jack. In addition to the increase in the amount of work involved, it is difficult to prevent voids from being created immediately below the hydraulic jack. Furthermore, it is difficult to solidly connect the two parts of the rebar cage below and above the hydraulic jack with each other.
Also, this device is not applicable to bored precast piles which are staked into the ground as the earthen shafts are bored with an auger or other means.